why do people expect facial expressions when you are doing nothing that requires them?

You should look into something called "modal action patterns."

You can look at someone who is smiling in our culture, and it translates to all other cultures as someone who is happy. These actions we make as a species, regardless of our upbringing, allow us to see what behaviors are inborn and not learned. Such as laughing when we find something humorous, smiling when we are happy, and frowning when we are upset.

So if someone told you something and expected an emotional response, and you did not communicate it, they may feel a little stifled. If you feel that your face does not communicate it, then you made need to do so verbally, otherwise you may catch someone off guard.

I suppose it's just the 'normal' thing to do. Us humans tend to label what 'normal' is, but i don't think you can really define it. Who care's what other people think anyways, just live life and be yourself!